How Long Does A Scratched Eye Take To Heal?
There isn't anything much more painful than a scratch on the eye. But try to prevent it as much as you can, sometimes they just happen. And there is not yet anything to magically make the scratch disappear. You are just going to have to wait it out.
Scratches of the eye fortunately heal over very quickly. The surface layer of the eye called epithelium will take anywhere from a few hours up to 3 days to restore itself following a scratch - depending on how large the scratch is. After this completes, there still is some additional time until the eye is back to full strength.
The healing of the epithelium, however, is a very complex process. All sorts of signal molecules are involved to make sure that everything goes as planned.
The Epithelium of the Cornea
Our cornea exists in different layers. These layers all work together to protect the cornea and make sure that it stays transparent. After-all, the cornea needs to be transparent for us to be able to see.
On the very surface layer of the cornea is a thin layer of stacked cells called epithelial cells. These cells work similar to how our skin works - protecting everything else inside from the outside. The epithelium protects the cornea from all the bad guys that we encounter in our normal day to day life.
Epithelium on top of the cornea; Image by StemBook (CC BY 3.0) / modified from original
But similar to how the skin can become scratched, our cornea can become scratched as well. This can be accidental - such as you ran into a tree twig. Scratches can also happen as a byproduct of eye surgery. But either way, scratches are painful. The unprotected cornea is the most sensitive structure in our body.
So to get rid of all that pain and to prevent the unprotected cornea from becoming infected, we want this epithelium to heal over as quickly as possible.
Where Does the Epithelium Come From
The epithelium is actually in constant production and turn-over. This ensures that these cells remain healthy and transparent. Brand new epithelial cells are constantly replacing the damaged or “worn-out” ones.
These new cells come from the edge of our cornea at a position called the limbus. This is the ONLY spot in the cornea where new cells can be produced. The cells divide here and spread out to cover the cornea. This process speeds up when there is a scratch on the eye.
Eventually what needs to happen is new epithelium needs to replace the old damaged or missing epithelium. But this occurs in different phases and is highly coordinated by various growth factors and inflammation signaling molecules.
The Time From Scratch To Healing
Initially, nothing really happens (at least what you can “visibly see”). After a scratch, the cornea prepares itself for healing. The epithelial cells next to the scratch start to flatten and prepare to cover the defect. Next, these flattened epithelial cells extend to bridge the gap and close off the scratch. When this step is completed, the pain goes away and thus one may consider the scratched healed over.
One thing that is clear is that smaller scratches will heal over quicker than larger scratches. A smaller scratch simply has less distance to clear. And in fact, very small scratches can heal over in just a few hours.
When a scratch gets much larger and large segments of epithelium are missing (now known as an abrasion), it can take much longer to heal over. It can take anywhere from a day to 3-4 days depending on how large the abrasion is.
Time to get out the calculator
We can actually use math to sorta determine how fast the scratch will heal over!
The epithelium will heal at approximately 0.05mm per hour. Because this occurs from all sides of the scratch, it means that the diameter of the scratch will close at around 0.1mm per hour. When you add some extra lag time of 0 to 4 hours before the healing starts, you can come up with a rough estimate for how long things will take to heal over.
Let’s say you have a tiny scratch. The width may only be 0.1mm. Because the actual healing will only take about an hour, when you add some lag time, you can expect the epithelium to cover this scratch within a few hours.
Let’s look at a large abrasion. Someone who has had the laser eye surgery PRK will have a surgically created abrasion of around 7-8mm. This will take longer to heal up. After some lag time, you are looking at a little over 3 days for the epithelium to cover the cornea.
At this point, most people will consider the scratch healed over. Because the epithelium covers the cornea, most of the pain will be gone.
So we're done right? Well, not so fast.
How Long Until Everything is Permanently Healed
After the epithelium covers the cornea, it must build up a few more layers. The epithelium isn’t just a single layer of cells. It is about 5-7 layers thick. As the limbus produces more cells, these cells push in and build up the layers of cells. This build-up of layers will take about another 24 hours to complete.
Finally we have one more important step. The epithelium must bind to the rest of the cornea below. If this doesn’t occur, the epithelium can be easily brushed away and the whole process will need to be repeated. This step can be by far the longest step.
Normally the epithelium is attached to something called the basement membrane. If this membrane isn’t injured, the epithelium can rapidly attached and return to full strength within a week. But if this membrane is damaged, it can take over a month for the full strength of the epithelium to return.
So avoid rubbing your eyes when you have a scratch to avoid having to repeat the whole healing process over again.
Not All Scratches Will Heal At The Same Rate
Certain things can interfere with the normal healing process and cause scratches to take much longer to heal up.
- There isn’t an adequate amount of limbus cells to produce new epithelium. This can occur after injuries to the eye such as chemical burns.
- The epithelium can’t attach to the cornea very well. This occurs in a certain corneal condition called basement membrane dystrophy.
- There is an abundance of inflammation. This can happen in conditions with severe dry eye.
- There is poor nerve function causing dysfunction of the epithelium. Conditions such as diabetes and certain viral infections of the eye can contribute to this.
- An active infection in the cornea will really mess up everything and prevent healing until the infection is under control.
These lead to persistent scratches or persistent epithelial defects which must be treated and monitored closely with an eye doctor to heal without any complication.
What is the biggest concern of a scratch? The concern is an infection can develop. A scratch causes a break-down in the protective layer of the cornea. Thus, bacteria are free to enter and grow. Infections will not only prevent the scratch from healing up, infections also come with the risk of scarring in the cornea. These scars disrupt the clear and transparent properties of the cornea and can cause permanent vision loss.
Thus, while a scratch is healing up, antibiotics are a good idea to prevent any infections from developing. Antibiotics are essential for large scratches or abrasions which will take longer to heal up and for scratches caused by something unclean - such as a tree branch, pet scratch, etc.
A scratch on the eye will fortunately heal over very quickly. The pain from a scratch will go away after a few hours to a few days depending on the size of the scratch. Afterwards, it will still take some time for the epithelium to fully heal up and develop its permanent attachments.
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